These pages make the localization of www.gnu.org complete, so you may want to translate them even though they are not on the philosophical priority list.
The first one is gnu-404.html. This is the page shown when the visitor encounters a broken link; it explains the situation and suggests a few frequently requested pages.
The second page is server/select-language.html. It explains how the language negotiation works (see language-negotiation) and provides a way to customize it to some extent.